Fabrication Of Microlens Arrays Research Articles

Fabrication of microlens array on 6H-SiC mold by an integrated microcutting-etching process

Precision glass molding (PGM) is an efficient method to mass-produce glass microlens arrays with high-precision, the key to which is the high-quality mold. Silicon carbide (SiC) is considered to be an ideal mold material, but microstructures are difficult to machine in SiC because of its high hardness and strength. In this study, a microcutting-etching method involving the combination of single-point diamond cutting with inductively coupled plasma (ICP) etching is proposed to fabricate microlens arrays on 6H-SiC. The research described in this paper focuses on the use of the proposed method for controllable processing of a microlens array mold made of 6H-SiC. The microcutting process of the mask material and the etching process of the substrate material 6H-SiC are studied. The techniques to control the accuracy of the microlens array on 6H-SiC are managed by studying the cutting process, selectivity ratio and etching condition. In this manner, a ring-shaped microlens array with excellent accuracy and consistency is generated.

3D fabrication of spherical microlens arrays on concave and convex silica surfaces

Three-dimensional (3D) microlens arrays are a group of micro-optics structures on curved surfaces that offer unique optical functions such as wide field of view in a compact arrangement. This paper reports an improved rapid lapping and molding system for 3D fabrication of spherical microlens arrays on non-planar surfaces including concave and convex surfaces for optical applications. Unlike traditional approaches for 3D micro patterns on curved silicon/silica substrates, this research demonstrates a low-cost and efficient chemical mechanical polishing process by lapping precision microlenses with steel balls and diamond slurries. Different from lapping on plane surfaces, lapping parameters for each micro cavity need to be accurately calculated and controlled to obtain microlenses with same apertures. Therefore, a micro wear model for micro cavity lapping process was established to calculate cavity sag height with the knowledge of down force, relative velocity and lapping time. Several groups of microlenses lapping were then conducted under the same conditions to validate the micro wear model. Guided by the micro wear model, two groups of microlens arrays were fabricated on concave and convex surfaces respectively. The shape accuracy and surface texture of the microlens arrays were evaluated by using a white light interferometer. Afterwards, the silica molds were coated with graphene film and then utilized to copy the 3D microstructures onto polymers by rapid surface molding. The wide angle imaging characterization of the 3D polymeric microlens arrays was illustrated by a simple optical setup and the measured MTF curves using the slanted-edge method. The improved manufacturing platform demonstrates a new approach for 3D microlens arrays fabrication on hard substrates in a cost effective way.

Fabrication of microlens array with well-defined shape by spatially constrained thermal reflow

We demonstrate a novel method for fabricating a well-defined and large-area microlens array combining digital micro-mirror device (DMD)-based lithography and spatially constrained thermal reflow. Using the flexible processing ability of DMD-based lithography, a micro pillar array is easily prepared on a glass substrate and then the microlens array is formed after a thermal reflow process. Differing from conventional approaches, the proposed thermal reflow process is conducted in polydimethylsiloxane (PDMS) solution instead of in the air. During the process, the partly cured PDMS can effectively prevent the photoresist crawling in the diameter direction, which is very convenient for controlling the precise shape of the microlens. In addition, the photoresist microlens array and PDMS soft mold with a concave cavity array can be achieved in one step. The experimental results showed that the photoresist microlens was good in both shape and surface quality. The presented method may offer great promise for fabricating a high-quality microlens array with different requirements in optical applications.

Fabrication of an infrared Shack–Hartmann sensor by combining high-speed single-point diamond milling and precision compression molding processes

A novel fabrication method by combining high-speed single-point diamond milling and precision compression molding processes for fabrication of discontinuous freeform microlens arrays was proposed. Compared with slow tool servo diamond broaching, high-speed single-point diamond milling was selected for its flexibility in the fabrication of true 3D optical surfaces with discontinuous features. The advantage of single-point diamond milling is that the surface features can be constructed sequentially by spacing the axes of a virtual spindle at arbitrary positions based on the combination of rotational and translational motions of both the high-speed spindle and linear slides. By employing this method, each micro-lenslet was regarded as a microstructure cell by passing the axis of the virtual spindle through the vertex of each cell. An optimization arithmetic based on minimum-area fabrication was introduced to the machining process to further increase the machining efficiency. After the mold insert was machined, it was employed to replicate the microlens array onto chalcogenide glass. In the ensuing optical measurement, the self-built Shack-Hartmann wavefront sensor was proven to be accurate in detecting an infrared wavefront by both experiments and numerical simulation. The combined results showed that precision compression molding of chalcogenide glasses could be an economic and precision optical fabrication technology for high-volume production of infrared optics.

Design and fabrication of ultrathin lighting responsive security device based on moiré imaging phenomenon

Security authentication of high- and low-value products is a pressing issue. Security technologies should be upgraded before they are counterfeited. In this study, an ultrathin lighting responsive moiré imaging device, which is different from traditional security device is developed for security authentication. Some interesting lighting responsive moiré imaging phenomena including light-activated three-dimensional floating imaging and light-controlled image transformation have been developed as covert security features for advanced authentication. These security features can be verified instantly using a point light source such as a white light-emitting diode configured in a smart phone. A compact defocus optical structure consisting of a micro lens-mirror array (MLMA) and a micro-pattern array is designed to fabricate the lighting responsive moiré imaging device. The structure makes the device ultrathin, which enables the easy integration of the device. The MLMA also overcame the difficulty in the fabrication of high numerical aperture micro-lens array required in traditional moiré imaging system. Flexible devices with 78-μm thickness were fabricated by photolithography, imprinting, and gravure-like techniques. Such thin device allows its potential integration with banknotes, currency, or other valuable documents/products.

Conductive and transparent submicron polymer lens array fabrication for electrowetting applications

In this study, we have demonstrated a simple and inexpensive process to fabricate electrowetting lens arrays with various curvatures (micron to submicron) on conductive and transparent polydimethylsiloxane (PDMS) molds without additional metal layers. The microlens arrays were fabricated using one-step dual diffuser lithography process, which utilizes a pair of diffusers to diffract the incident rays of UV light at wide angles before approaching the photoresist. Dimensional control and high fill factor was achieved by just varying the exposure energy and gap between the patterns in the photomask, respectively. The patterns were replicated in conductive and transparent Ag(n)-PDMS (5–20% Ag) with 15 μm thickness. High conductivity of 4.6 × 10−1 S/m and high transmission efficiency of 90% was demonstrated by Ag(n)-PDMS molds. Micro-nanolens arrays fabricated by the optimized corelation were utilized to demonstrate switchable wettability behavior of water droplet at different applied voltages. The electrowetting microlens array fabrication method introduced in this work has high potential to be incorporated in optoelectronics and biomedical devices.

Fabrication of spherical microlens array by combining lapping on silicon wafer and rapid surface molding

Silicon is a promising mold material for compression molding because of its properties of hardness and abrasion resistance. Silicon wafers with carbide-bonded graphene coating and micro-patterns were evaluated as molds for the fabrication of microlens arrays. This study presents an efficient but flexible manufacturing method for microlens arrays that combines a lapping method and a rapid molding procedure. Unlike conventional processes for microstructures on silicon wafers, such as diamond machining and photolithography, this research demonstrates a unique approach by employing precision steel balls and diamond slurries to create microlenses with accurate geometry. The feasibility of this method was demonstrated by the fabrication of several microlens arrays with different aperture sizes and pitches on silicon molds. The geometrical accuracy and surface roughness of the microlens arrays were measured using an optical profiler. The measurement results indicated good agreement with the optical profile of the design. The silicon molds were then used to copy the microstructures onto polymer substrates. The uniformity and quality of the samples molded through rapid surface molding were also assessed and statistically quantified. To further evaluate the optical functionality of the molded microlens arrays, the focal lengths of the microlens arrays were measured using a simple optical setup. The measurements showed that the microlens arrays molded in this research were compatible with conventional manufacturing methods. This research demonstrated an alternative low-cost and efficient method for microstructure fabrication on silicon wafers, together with the follow-up optical molding processes.

Fabrication method of quartz aspheric microlens array for turning mask

In order to solve the two difficult problems of the poor processing controllability and the low surface accuracy of quartz aspheric microlens array processing, a fabrication method of quartz aspheric microlens array for turning mask is proposed. This method mainly uses single point diamond turning technology and reactive ion etching technology, studies the turning and etching properties of the mask material, and optimizes the mask material by experiment. Finally, the fabrication of an aspherical glass microlens array with an area of 5 mm×5 mm was carried out. The experimental results are compared with the expected parameters. The analysis shows that the error root mean square of the quartz glass component is 1.155 nm, and the surface accuracy error is 0.47%.

Fabrication of parabolic cylindrical microlens array by shaped femtosecond laser

A simple and efficient technique for fabricating parabolic cylindrical microlens arrays (CMLAs) on the surface of fused silica by shaped femtosecond (fs) laser direct-writing is demonstrated. By means of spatially shaping of a Gaussian fs laser beam to a Bessel distribution, an inversed cylindrical shape laser intensity profile is formed in a specific cross-sectional plane among the shaped optical field. Applying it to experiments, large area close-packed parabolic CMLAs with line-width of 37.5 μm and array size of about 5 × 5 mm are produced. The cross-sectional outline of obtained lenslets has a satisfied parabolic profile and the numerical aperture (NA) of lenslets is more than 0.35. Furthermore, the focusing performance of the fabricated CMLA is also tested in this work and it has been demonstrated that the focusing power of the CMLA with a parabolic profile is better than that with a semi-circular one.

Square-Aperture Microlens Array Fabrication by the Moving Mask Technology

Square-Aperture Microlens Array Fabrication by the Moving Mask Technology

Square-Aperture Microlens Array Fabrication by the Moving Mask Technology

Square-Aperture Microlens Array Fabrication by the Moving Mask Technology

Fabrication of polymer microlens array with controllable focal length by modifying surface wettability.

A straightforward technique for fabricating low-cost microlens arrays with controllable focal length is developed. By harnessing and manipulating the interfacial energy between the liquid-state acrylate resin and the solidified polydimethylsiloxane (PDMS), the surface of the acrylate resin in the PDMS microhole presents a spherical shape and the curvature can be flexibly controlled. With the change of the processing time for the surface modification of the PDMS microholes, the focal length of the concave microlenses varies from -296.3 μm to -67.4 μm. The numerical aperture of 0.45 is realized. The focal length and the aperture of the microlenses are also affected by the diameter of the microholes. The fabricated concave microlens array can be employed as a master to further duplicate convex microlens array. A good image quality can be achieved by using the convex microlens arrays.

Fabrication of microlens array with controllable high NA and tailored optical characteristics using confined ink-jetting

This work presents an economic and controllable fabricating method of high numerical aperture (NA) polymer microlens array (MLA) based on ink-jetting technology. The MLAs are ink-jetted to align on micro platforms patterned flexible PDMS substrate. The shape of a sole lens is constructed by the ink-jetted pre-cured polymer volume confined on a micro platform. In this way, MLAs with targeted geometries—as well as tailored optical characteristics—can be printed, leading to freely designed optical properties. High NA from 0.446 to 0.885 and focal lengths between 99.26 μm and 39.45 μm are demonstrated, confirming theoretical predictions. Particularly, both the simulations and experimental measurements in optical properties are carried out, demonstrating that microlenses with shapes beyond a hemisphere (CA > 90°) exhibits higher light utilization efficiency and wider viewing angle. Meanwhile, the MLAs are fabricated on flexible PDMS substrates and can be attached to other curved surfaces for wider field of view imaging and higher sensitivity.

Investigation of the occupancy ratio dependence for microlens arrays on diamond.

Diamond microlens arrays with a high occupancy ratio were fabricated by an improved thermal reflow method. Our resultes show that the occupancy ratio of a photoresist mask could be improved with optimizing the reflow temperature, time and photoresist thickness during the reflow process. The fabricated microlens arrays exhibited a uniform arrangement and good optical performance.

Direct laser writing for micro-optical devices using a negative photoresist.

Direct laser writing (DLW) via two-photon absorption (TPA) has attracted much attention as a new microfabrication technique because it can be applied to fabricate complex, three-dimensional (3D) microstructures. In this study, 3D microstructures and micro-optical devices of micro-lens array on the micrometer scale are fabricated using the negative photoresist SU-8 through TPA with a femtosecond laser pulse under a microscope. The effects of the irradiation conditions on linewidths, such as laser power, writing speed, and writing cycles (a number of times a line is overwritten), are investigated before the fabrication of the 3D microstructures. Various microstructures such as woodpiles, hemisphere and microstructures, 3D micro-lens and micro-lens array for micro-optical devices are fabricated. The shape of the micro-lens is evaluated using the shape analysis mode of a laser microscope to calculate the working distance of the fabricated micro-lenses. The calculated working distance corresponds well to the experimentally measured value. The focusing performance of the fabricated micro-lens is confirmed by the TPA fluorescence of an isopropyl thioxanthone (ITX) ethanol solution excited by a Ti:sapphire femtosecond laser at 800 nm. Micro-lens array (assembled 9 micro-lenses) are fabricated. Nine independent woodpile structures are simultaneously manufactured by DLW via TPA to confirm the multi-focusing ability using the fabricated micro-lens array.

Fabrication of high quality aspheric microlens array by dose-modulated lithography and surface thermal reflow

A novel fabrication method for high quality aspheric microlens array (MLA) was developed by combining the dose-modulated DMD-based lithography and surface thermal reflow process. In this method, the complex shape of aspheric microlens is pre-modeled via dose modulation in a digital micromirror device (DMD) based maskless projection lithography. And the dose modulation mainly depends on the distribution of exposure dose of photoresist. Then the pre-shaped aspheric microlens is polished by a following non-contact thermal reflow (NCTR) process. Different from the normal process, the reflow process here is investigated to improve the surface quality while keeping the pre-modeled shape unchanged, and thus will avoid the difficulties in generating the aspheric surface during reflow. Fabrication of a designed aspheric MLA with this method was demonstrated in experiments. Results showed that the obtained aspheric MLA was good in both shape accuracy and surface quality. The presented method may be a promising approach in rapidly fabricating high quality aspheric microlens with complex surface.

A simple method for shape modulation in microlens array fabrication via spin-coating process

In this paper, a simple method for microlens array (MLA) fabrication is presented by the modulation of spin-coating process. Firstly, micro holes with design diameters were prepared on substrate through maskless lithography based on the digital micromirror device. Next by the spin-coating of photoresist onto the substrate the meniscus concave microlens with various curvatures can be formed, and then become stable after a proper baking process. During the coating process, the photoresist was strictly confined in the micro holes and its volume could be precisely controlled by adjusting the spin-coating speed. These advantages make it very easy to obtain microlenses with different aspect ratios at precisely pre-defined diameters. Experimental results showed the effectiveness of our method. The presented method is expected to provide a novel, economic and simple strategy for shape modulation in MLA fabrication.

Highly uniform manufacturing method for large-area microlens arrays

The microlens array is a key microstructure component in imaging systems, such as in light field cameras. A resolution of the light field camera can facilitate the use of a small-area microlens array. However, fabricating a large-area microlens array with a highly accurate and consistent surface finish is still difficult and high-cost. A low-cost manufacturing method is proposed in this study, where the mould core of the microlens array is fabricated with high precision and high uniformity by cylindrical ultra-precision diamond turning. The proposed tool path strategy is employed to achieve superior surface quality and avoid the dynamic vibration of an unsmooth path. Light field camera prototypes are developed using a commercial DSLR camera with a fabricated microlens array. The successful performance of the prototypes confirms that the proposed manufacturing method satisfies the application demands of large-area microlens arrays.

Fabrication of Microlens Arrays with Controlled Curvature by Micromolding Water Condensing Based Porous Films for Deep Ultraviolet LEDs

Microlens arrays (MLAs) have attracted wide attention due to their crucial applications in optics, optoelectronics, and biochemistry. In this paper, we present a simple and green approach for the economical fabrication of MLAs with controlled curvature based on water condensing. By controlling the input current and working time of initiative cooling and the viscoelasticity of UV-curable polymer, uniform porous films with adjustable morphology were prepared. MLAs with aspect ratios of 1.41, 1.01, and 0.69 were fabricated by micromolding the porous film templates. Furthermore, the fluoropolymer encapsulations with the MLAs were applied for the packaging of deep ultraviolet light-emitting diodes (DUV-LEDs). Consequently, the light output powers of DUV-LEDs are enhanced by 7.1%, 10.2%, and 15.4%, respectively, by using these MLAs at the driving current of 350 mA.

Fabrication of Hydrophobic Micro-lens Arrays by Capillary Force Lithography

Fabrication of Hydrophobic Micro-lens Arrays by Capillary Force Lithography